Creating 3D Geometry - AutoCAD 2011 and AutoCAD LT 2011: No Experience Required - page 900

FiGuRE 16.87 Move the boxes so they protrude through the roof.
6. If you subtract both roof objects at the same time, the resultant roof
object will be a single entity. Use this procedure to cut the boxes out
of the roof.
a. Copy the boxes in-place using the same point as the base point
and the displacement. This can be done by pressing ↵ at the
Specify base point and Specify displacement prompts.
b. Select the roof surface and subtract one set of boxes from it.
c. Select the roof deck and subtract the second set of boxes from it.
When you are done, the roof should look like Figure 16.88.
7. Hold the Ctrl key down and click the middle of the outer wall of
the pop-out. You may have trouble selecting the proper edge when
both of the adjacent faces are visible. If you encounter a problem, try
selecting the edge from a northeastern viewpoint.
8. Turn on Ortho mode; then move the edge downward, below the
surface of the roof. Figure 16.89 shows the edge being moved from a
northeastern viewpoint.
9. Freeze and unlock all the 2D layers (including those with an A-ELEV
prefix) and thaw the 3D layers (with a -3DOB suffix).
10. Drag the ViewCube to change to an isometric view, and then adjust
the height of the roof support posts so that they extend into the roof
deck but not through the roof surface (see Figure 16.90).
11. Your completed cabin should look like Figure 16.91.
12. Save your file as 16A-3DMOD6.dwg .